Soil aggregation alterations under soil microplastic and biochar addition and aging process

Soil microplastics (MPs) are a substantial threat to soil health, particularly by disrupting soil aggregation. Additionally, MPs undergo aging processes in the soil, which may significantly alter their long-term impacts on soil structure. To investigate these effects, we conducted an eight-month soil incubation experiment, examining the influence of MPs and their aging on soil aggregation. The experiment utilized a factorial design with various combinations of MPs and biochar additions: 1% by weight of 1000-mesh polyethylene and polypropylene MPs, and 5-mm biochar, resulting in six treatment groups: [CK], [PE], [PP], [Biochar], [PE + biochar], and [PP + biochar]. Our findings revealed that both MPs and biochar underwent aging throughout the incubation, evidenced by the formation of oxygen-containing functional groups on their surfaces. Microplastics, particularly polyethylene, primarily affected the 0.5–1 mm and >2 mm aggregate fractions, with average reductions of 21% and 77%, respectively. These adverse effects intensified with the aging of MPs. Contrary to expectations, the addition of biochar was found to exacerbate the negative impacts of MPs on the 0.25–0.5 mm aggregates, with a decrease of 11% associated with PE MPs. The influence of biochar on mitigating the damage caused by MPs to soil aggregation is dependent on aggregate size.